The industrial and commercial use of the three phase multitorque electric motor has an almost infinite number of applications. Among these applications of the three phase electric motor, there may be found many instances where the torque load requirements of the output shaft vary over a sustained period of time. Examples are cooling fans, pumpjack units, pipeline gathering systems and conveyor belts, where varying load requirements are encountered. The electric motors in these and other examples must be sized to accommodate the largest anticipated load requirement. Accordingly, many times during an operational period, only a fraction of the motor torque capability is required. During the period of time when less than the available torque is utilized, electric energy is wasted.
In recent years, energy in all forms is becoming progressively expensive and, perhaps, limited. Therefore, the time has arrived for new concepts of operation of the multitorque motor to be examined. One solution to some of the problems discussed above is the proper application of the multi-torque, three phase, constant voltage, electric motor.
These multi-torque motors have three field coils. Each of the three coils is comprised of dual windings so that nine leads exit from the motor, thereby enabling the windings to be connected in Y, .DELTA. Y, or .DELTA. configuration, respectively; thus providing a selection of low, medium, and high running torques, respectively.
Upon initial installation of the multitorque motor, a selection of the electrical connection of the nine electrical leads connected to the various windings must be mechanically effected at that time, and should it subsequently be desired to change the winding connections from .DELTA. to .DELTA. Y, for example, thereby reducing the available torque or power, it is necessary for the electrician to visit the electrical installation, break the old connections, and make the necessary changes. Should these changes from .DELTA. to .DELTA. Y subsequently prove to be more than adequate as a prime mover for the load requirement, the motor will run under unnecessarily high torque conditions. Moreover, there still remains the unused and more desirable low torque Y connection, which is the most desirable mode of operation when the load conditions permit this configuration.
Accordingly, anytime the motor torque mis-matches the requirements of the load, it is necessary to recognize the undesirable condition and to instruct an electrician to mechanically effect the required changes. This manual changing of the winding is impractical for the examples cited above, i.e., cooling fans, pumpjacks, pipeline gathering systems, and conveyor belts because of the numerous changes in the winding configuration which must be effected from time to time. Therefore, it is expected that the motor will not always be operating in its most efficient torque mode; and, in an extreme example, the motor often overheats and is shut down if severely overloaded.
Accordingly, it would be desirable to have made available a means by which the stator windings of a multi-torque motor is automatically connected in the configuration which provides the torque best suited for the particular load requirement. An automatic load seeking control, which continually provides this selection is the subject of the present invention.